> what is the data that is worth backing up in a really futureproof manner?

A futureproof encoding would have to be something like images at a scale visible to the naked eye engraved in a substance that cannot be corroded or destroyed. Stainless steel is probably fine for the material, but you run into limits on the amount of space (and steel) you're willing to devote to the project. And most information just can't be represented pictorially anyway, so the theoretical possibility of futureproof storage isn't even there.